Papermaking fabrics, forming wires, webs or fabrics for coated, impermeable bands and belts for extended nip presses and fabrics for forming roll coatings have to fulfill extremely strong requirements. Therefor, for the manufacturing of such fabrics and webs only specific materials can be used which fulfill these requirements.
Thus, fabrics for forming wires are manufactured of polymeric monofiles using for the longitudinal threads exclusively polyester-monofiles, because only with that kind of monofile the required stability in longitudinal direction can be gained. As under operation longitudinal stresses of more than 200 N/cm. width of the wire may occur, the essentially abrasion resistant or wear resistant polyamide, for instance, cannot be used as material for the longitudinal threads, because this material is too highly stretching, so that such kind of wires would not have the required stretching or elastic stability.
On the other hand pure polyester wires, i.e. forming wires in which the cross threads, too, consist of polyester monofiles have no sufficient life time, because polyester has in comparison to polyamide an essentially lower abrasive resistance, so that the operational time of those wires is much too short in order to use these wires as useful alternative. This drawback is also not balanced by the fact that pure polyester wires having the known common double layered binding require as shown by tests an energy input which is in comparison to the so called mixed wires of the same kind, consisting of polyester as well as also of polyamide, by about 30% smaller.
Moreover, it was found out that wires the cross threads thereof consist exclusively of polyamide are not sufficiently stable in cross direction because of the low configurational resistance at the high water absorption, causing bulging of the monofiles. This deficiency results therein that such wires on running on the paper machine form pleats.